1. Field of the Invention
The present invention relates to an apparatus and method for facilitating the respiration of a patient and is particularly useful in treating disturbed breathing, snoring, mixed obstructive sleep apnea, and certain cardiovascular sleep conditions. More particularly, the present invention is concerned with an apparatus and method for imposing a positive pressure on the patient's airways just prior to the onset of inhalation in order to induce and/or permit inhalation, and for subsequently reducing the pressure on the airways to ease exhalation effort. Another aspect of the invention is concerned with monitoring sounds associated with patient's respiration and controlling the gas pressure delivered to the patient's respiratory passages in accordance with the sounds.
2. Description of the Prior Art
Obstructive sleep apnea is a sleep disorder characterized by relaxation of the airway including the genioglossus throat muscle tissue during sleep. When this occurs, the relaxed muscle can partially or completely block the patient's airway, a condition more prevalent in overweight patients. Partial blockage can result in snoring. Complete blockage can result in sleep apnea.
When complete blockage occurs, the patient's inhalation efforts do not result in the intake of air and the patient becomes oxygen deprived. In reaction, the patient begins to awaken. Upon reaching a nearly awakened state, the genioglossus muscle resumes normal tension which clears the airway and allows inhalation to occur. The patient then falls back to a deeper sleep whereupon the genioglossus muscle again relaxes and the apneic cycle repeats.
Central apnea is when no inspirator effort occurs or is delayed. Central apnea may be combined with obstructive apnea, known as mixed apnea. Other breathing irregularities such as Cheynes Stockes breathing may have apneic intervals when intake airflow ceases.
In some patients, sleep apnea events can occur dozens of times during the course of a sleep session. In consequence, the patient never achieves a fully relaxed, deep sleep session because of the repetitive arousal to a nearly awakened state. The patient is also deprived of REM (rapid eye movement) sleep. People afflicted with sleep apnea are continually tired even after an apparently normal night's sleep.
In order to treat obstructive sleep apnea, the so-called continuous positive airway pressure (CPAP) system has been devised in which a prescribed level of positive airway pressure is continuously imposed on the patient's airways. The presence of such positive pressure on the airways provides a pressure splint to offset the negative inspiratory pressure to maintain tissue position tension and thereby maintain an open patient airway. The positive airway connection with a patient is typically achieved by way of a nasal pillow such as that disclosed in U.S. Pat. No. 4,782,832 hereby incorporated by reference in which the nasal pillow seals with the patient's nares and imposes the positive airway pressure on the nasal passages.
The CPAP system meets with objections from patients, however, because the patient must exhale against the positive pressure. This increases the work to exhale. Some patients have difficulty getting used to this and as a result, may discontinue the therapy. Drying of the nose and airway due to continuous circulation of room air is also a complaint. Also, exhaled carbon dioxide tends to remain in some nasal masks with CPAP therapy.
In prescribing CPAP therapy, it is usually necessary for a patient to spend one or two nights in a sleep treatment laboratory where it is first determined whether the patient has a respiratory disorder such as sleep apnea. If so, the patient is then fitted with a CPAP device whereupon the required gas pressure is determined for providing the necessary air splint to maintain airway patency.
The required pressure for maintaining patency is usually higher when the patient is sleeping on his or her back than when sleeping in a side rest position. The higher pressure is usually prescribed in order to ensure sufficient pressure in all sleeping positions. The higher pressure is not needed, however, in all circumstances. For example, before the patient has fallen asleep and in the early stages of sleep, the higher pressures are not needed. Additionally, the higher pressures are often not needed during deep sleep when the patient is in the side rest position. Furthermore, a given patient may only be subject to sleep apnea under certain conditions such as when the patient is extremely tired or under the influence of alcohol or sleep-inducing drugs. As a result, the patient is subjected to the discomfort of the high prescription pressures even when not needed.